The present invention relates to resistors, and more particularly, this invention relates to tunnel junction resistors used with semiconductor and magnetic storage systems.
In magnetic storage systems, data is read from and written onto magnetic recording media utilizing magnetic transducers commonly. Data is written on the magnetic recording media by moving a magnetic recording transducer to a position over the media where the data is to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic media. Data is read from the media by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic media. Read and write operations may be independently synchronized with the movement of the media to ensure that the data can be read from and written to the desired location on the media.
An important and continuing goal in the data storage industry is that of increasing the density of data stored on a medium. For tape storage systems, that goal has lead to increasing the track density on recording tape, and decreasing the thickness of the magnetic tape medium. However, the development of small footprint, higher performance tape drive systems has created various problems in the design of a tape head assembly for use in such systems.
Circuits using semiconductor devices and magnetic storage devices utilize inductive writers and/or readers which are fabricated on the wafer to write data to and read data from the magnetic storage media. These writers and readers can be either external to the wafer or can be thin film resistors deposited onto the wafer. The thin film readers on a wafer are sensitive to electrostatic discharge (ESD) damage.
The readers typically include magnetoresistive (MR) sensors comprising thin film sheet resistors that are highly susceptible to damage from ESD, either through Joule heating from high currents or from dielectric breakdown. The writers are inductive, and alone are much less susceptible to ESD damage from high currents because they are built to sustain high writer currents. However, a typical HDD or tape MR transducer head comprises a piggyback structure, wherein components of each reader and writer are separated by one or more relatively thin insulation layers such as oxide layers. The reader and writer are encapsulated by a substrate and a closure. The writer is stacked above or below the reader vertically in thin film layers.
The insulation layers between the readers and writers are susceptible to dielectric breakdown with damaging electric field levels on the order of 1×108 to 2×108 V/m. One specific failure mode that takes place in the piggyback structured head is shorting between the reader and writer within the same transducer element. For an insulation layer thickness of 0.6 microns, a voltage differential of 60-120 V will result in dielectric breakdown leading to ESD failure and resulting damage. Furthermore, ESD damage to GMR and TMR sensors can occur with voltages as low as 0.5 to 1 V.
ESD damage is a detractor for production yield during the transducer head manufacturing process. ESD damage can manifest in MR sensor resistance value as over high limit (OHL), as under low limit resistance (ULL) measurement and any value in-between. Subtle ESD damage can also be magnetic in nature and may not be readily observable as a change in resistance. The likelihood of shorting events between readers and writers due to ESD can be as high as the typical OHL failure mode. Extant tape heads contain upwards of 30 to 40 reader-writer pairs per tape head, such that a per-transducer loss as low as 0.1% translates to a large loss of 3 to 4% loss of tape heads.
One approach to preventing ESD damage is to connect neighboring reader and/or writers together to maintain an equipotential. However, space on the device is at a premium, and most devices do not have room for a shunt having the appropriate resistance. This is particularly so in a device having multiple readers and/or writers, such as in a tape head. For example, for common metals used for thin film resistors, resistances of 10 kΩ and higher utilize large aerial real estate on the wafer. For a standard thin film resistor, the resistance is given by:
                    R        =                              ρ            *            L                                H            *            D                                              Equation        ⁢                                  ⁢        1            where ρ is the material resistivity, L is the length of the film, H the height, and D the thickness of the material. Taking Tantalum (Ta) as an example, with a resistivity of ρTA=13×10−8 Ω-m, and a value of 100 nm for D, a 100 nm for H, for a resistance of 100 kΩ, the value of L would be 7.7 mm. Even dropping H and D to 10 nm each, L would be 77 mm. For many situations, this length utilizes a large amount of area on a wafer. Furthermore, the long leads could act as an antenna to adversely couple into radio frequency (RF) signals. Thin film, high impedance, resistors are used in MR heads used for tape and disk drive storage. A major problem for the tape heads is the large amount of a real space taken up by the thin film, high impedance resistors. Therefore, a resistor which occupies less a real space for use with high resistance devices is desirable.